The phrase "can't put something down" is often used to describe a situation where someone is completely engrossed in or addicted to something. Synonyms for this phrase could include "captivated," "enchanted," "mesmerized," "obsessed," "hooked," "enthralled," "enthused," "spellbound," "immersed," "consumed," "absorbed," or "fixated." Each of these words conveys a sense of intense focus and fascination, and can be used when describing a variety of activities, from reading a captivating book to playing an addictive video game. Regardless of the context, these synonyms all express a sense of being completely absorbed in something to the point where it's difficult to stop.